Many high-speed wireless networks use wideband modulation techniques. Examples include multi-carrier communication systems, such as an orthogonal frequency division multiplexing (OFDM) system or an orthogonal frequency division multiple access (OFDMA) system, as well as spread spectrum systems based on various code division multiple access (CDMA) and Wideband CDMA (WCDMA) standards.
Wireless communication devices implement a power amplifier to amplify a transmit signal waveform prior to transmission. An ideal power amplifier would be a totally linear device. However, real power amplifiers are only linear within certain practical limits. Any real power amplifier device can be characterized by plotting a response curve that plots its input signal power versus output signal power. In most cases, the response curve is essentially linear over a certain range of input signal powers, but then transitions to a non-linear range. Some power amplifiers, such as traveling wave tube amplifiers, even exhibit nonlinear input to output characteristics prior to the compression region of the PA. With respect to power amplifiers in which a portion of the response curve is essentially linear, as the input signal power increases the power amplifier eventually begins to saturate. At this point, the response curve transitions into the non-linear range where the output signal power amplitude compresses and no longer increases linearly. Operation of the power amplifier in the non-linear range causes “clipping” of modulated transmission signal and distorts the modulated transmission signal. Thus, when a power amplifier device non-linearly amplifies transmit information sequences, traditional transmission methods may experience an increase in channel estimation errors due to distortion induced by the power amplifier and an increase in the out-of-band signal power due to the distortion induced onto the information signal by the power amplifier.
One characteristic of the waveforms used in systems described above is that they can have relatively large peak-to-average power ratios (PAPRs). For instance, in a multi-carrier communication system, a multi-carrier transmission is composed of a number of independent signals, where each signal is centered a different frequency (commonly referred to as “carrier” frequencies). In most multi-carrier communication systems, the signals are combined together as a vector. An inverse fast Fourier transform (IFFT) is usually performed on the vector to produce a discrete time domain signal which is converted to a continuous time domain signal and transmitted. When multiple independent complex valued signals residing on multiple carrier frequencies are summed this can result in a transmit signal that is characterized by a relatively large peak-to-average power ratio (PAPR). As used herein the term PAPR refers to the ratio of the peaks of the amplitude of the signal to the average amplitude of the signal.
When a modulated transmission signal waveform has a large PAPR, the transmission signal waveform will include a number of amplitude peaks that often exceed the transmitter's power amplifier (PA) capabilities. If the amplitude of the modulated transmission signal is too large the transmitter PA will saturate, and peaks in the transmitted signal waveform are cutoff or “clipped.” When the peaks of the modulated transmission signal are clipped this causes harmonic distortions in the transmit signal, and a significant amount of information is lost. Distortion can reduce the effectiveness of signal transmission from the transmitter to a receiver and can interfere with in-band and out-of-band communications. Such non-linear transmission may cause significant out-of-band interference (i.e., interference outside the signal bandwidth, such as in the adjacent channels and/or other user channels), and also may induce undesired in-band interference, which adds distortion to the transmitted information bits and also to the channel training information. Furthermore, improper synthesis of the channel training information may lead to further channel estimation errors at the receiver. Thus, non-linear amplification of high peak-to-average power ratio signals and improper channel training information design may, in the receiver, result in unacceptably high channel estimation errors on the receiver side and excessively high bit error rates and/or symbol error rates of received signals and can significantly degrade the receiver demodulated BER/SER.
Thus, one concern is how to reduce the PAPR of a multi-carrier transmission, and combat any remaining distortion that result due to the large PAPR.
Accordingly, in systems that use waveforms having relatively large peak-to-average power ratios (PAPRs), what are needed are methods, systems and apparatus that can reduce non-linear distortion prior to transmission, and subsequently estimate and cancel any remaining non-linear distortion after transmission. Such systems should reduce distortion noise and improve power efficiency in peak-limited channels. It would be desirable if these techniques can allow the transmit power amplifier to operate at low input backoff (IBO) levels in a highly energy efficient manner to increase battery life, while still achieving acceptable distortion cancellation at the receiver to provide high performance channel estimation and improve operation in multipath fading channels. Other features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.